In recent years, endoscopes have come to be widely used in both the medical field and in industry. Recently, endoscopes have been developed which do not require an inserted part and which avoid the pain associated with inserting an insertion part. In these endoscopes that are used in the medical field, a patient swallows an endoscope which has been miniaturized and placed within a capsule. For example, see Japanese Laid Open Patent Application No. 2001-91860 and published PCT application WO 01/65995 A2.
In the above-mentioned patent application, an objective lens and an illumination means formed of a luminescent diode are installed inside a near-hemispherical transparent cover. An object to be observed is illuminated by a luminescent diode and an objective lens forms an image of the object on an image sensor. The objective lens has a configuration wherein lens elements of positive and negative refractive power are combined to form a joined lens, and there is no disclosure about the depth of field of the objective optical system. The image sensor is of the CMOS type, and the objective lens consists of a biconvex lens element, and there is no disclosure about the depth of field of the objective optical system.
In these conventional examples, because an objective lens and an illumination body are fixed inside a near-hemispherical transparent cover and the curvature radius is the same between the central region of the field of view and the peripheral region of the field of view, there was an inconvenience in that the mucosa layer easily contacts the central region of the transparent cover, causing an obstruction in the field of view of the central region. Also, when the transparent front cover has a single radius of curvature (i.e., one defined by one-half the outer diameter of the capsule that is fitted to the transparent cover), the outer diameter of the capsule becomes excessive, making the capsule difficult to swallow and giving it low motility when within the body.